Fuel cell-atmospheric-pressure turbine hybrid system

ABSTRACT

A fuel cell-atmospheric-pressure turbine hybrid system uses the thermal energy of a cell exhaust gas discharged from an atmospheric-pressure, high-temperature fuel cell effectively, does not need any additional emergency protective device, and enables the use of lightweight, easy-to-process structural and piping materials to reduces the cost. 
     The fuel cell-atmospheric-pressure turbine hybrid system includes: a combustor  2  for burning an exhaust gas G 1  discharged from an atmospheric-pressure, high-temperature fuel cell  1 ; a turbine  3  in which a combustion gas G 2  discharged from the combustor  2  expands and the pressure of the combustion gas G 2  drops to a negative pressure; a compressor  4  for compressing an exhaust gas G 3  discharged from the turbine  3  to increase the pressure of the exhaust gas G 3 ; and a heat exchanger  5  for transferring heat from the high-temperature exhaust gas G 3  discharged from the turbine  3  to low-temperature air A to be supplied to the fuel cell  1.

TECHNICAL FIELD

The present invention relates to a fuel cell-atmospheric-pressureturbine hybrid system built by combining an atmospheric-pressure,high-temperature fuel cell and an atmospheric-pressure turbine andcapable of efficiently generating electric power.

BACKGROUND ART

Each of known hybrid systems of this kind disclosed in Patent documents1 and 2 includes a combination of a high-pressure fuel cell and a gasturbine for driving a generator.

Patent document 1: JP 8-45523 A (FIG. 1 and the specification)

Patent document 2: JP 10-12255 A (FIG. 1 and the specification)

The conventional hybrid system using a gas turbine combined with acompressor, and a high-pressure fuel cell that operates at a highpressure equal to or higher than the output pressure of the compressorhas the following problems. A small hybrid system has a small gasturbine, and a fuel cell contained in a high-temperature, high-pressurecontainer. Therefore, the hybrid system needs a protective devicecapable of properly carrying out a shutdown procedure and of discharginga high-temperature, high-pressure gas outside the system in anemergency. The protective device imposes a large cost load on the smallhybrid system. The hybrid system needs a differential pressure controlsystem and control techniques for limiting the variation of differentialpressure during emergency shutdown within an allowable range determinedon the basis of the structural strength of the fuel cell. Thedifferential pressure control system also increases the cost of thehybrid system. The high-temperature, high-pressure container and thehigh-temperature, high-pressure pipes also increases the cost of thehybrid system.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide a fuelcell-atmospheric-pressure turbine hybrid system built by combining anatmospheric-pressure, high-temperature fuel cell and anatmospheric-pressure turbine, capable of effectively using the thermalenergy of the exhaust gas discharged from the high-temperature fuelcell, not additionally needing an emergency protection device and havingsimple construction.

A fuel cell-atmospheric-pressure turbine hybrid system includes: acombustor for burning an exhaust gas discharged from anatmospheric-pressure, high-temperature fuel cell; a turbine in which acombustion gas discharged from the combustor expands and the pressure ofthe combustion gas drops to a negative pressure; a compressor forcompressing an exhaust gas discharged from the turbine to increase thepressure of the exhaust gas; and a heat exchanger for transferring heatfrom the high-temperature exhaust gas discharged from the turbine tolow-temperature air to be supplied to the fuel cell. The term“atmospheric pressure” signifies the pressure of the environment inwhich the system is installed and the term “negative pressure” signifiesa pressure lower than the atmospheric pressure.

Fuel and air interact through an electrolyte in the fuel cell togenerate power and the fuel cell discharges a high-temperature cellexhaust gas. The combustor burns the high-temperature cell exhaust gasand discharges a combustion gas. The turbine is driven by the combustiongas of a pressure approximately equal to the atmospheric pressure. Thecombustion gas expands and the pressure of the combustion gas drops to anegative pressure while the combustion gas flows through the turbine.The compressor raises the pressure of the exhaust gas discharged fromthe turbine. The expanded exhaust gas discharged from the turbine. Theheat exchanger transfers the heat of the expanded exhaust gas dischargedfrom the turbine to low-temperature air to be supplied to the fuel cell.Since the exhaust gas is supplied to the compressor after thetemperature of the exhaust gas has been thus decreased, the exhaust gascan be compressed at a high compression efficiency and thereby theefficiency of the gas turbine is improved. The air heated at a hightemperature by the heat of the exhaust gas discharged from the turbineis supplied to the fuel cell to increase power generation efficiency.The combination of the atmospheric-pressure, high-temperature fuel celland the atmospheric-pressure turbine enables the effective use of thethermal energy of the high-temperature cell exhaust gas discharged fromthe fuel cell, does not produce any high pressures in the system, makesan additional emergency protective device unnecessary, and enables theuse of lightweight, easy-to-process structural and piping materials toreduces the cost.

Preferably, the exhaust gas discharged from the compressor is mixed inthe air to be supplied to the fuel cell. Particularly, when the fuelcell is a molten carbonate fuel cell (MCFC), the exhaust gas can besupplied to the fuel cell by a low-power blower or the like instead ofcompressing and supplying the exhaust gas to the fuel cell by a recycleblower. Therefore, the partial pressure of carbon dioxide around thecathode can be easily increased and power generation efficiency can beimproved even under an operating condition where cathodic reaction rateis low.

Preferably, a cooler is disposed below the heat exchanger to cool anexhaust gas discharged from the heat exchanger. The respectiveefficiencies of the compressor and the gas turbine can be improved bysupplying the exhaust gas discharged from the heat exchanger to thecompressor after cooling the exhaust gas by the cooler.

A preferred embodiment of the present invention includes a secondcompressor disposed coaxially with the compressor serving as a firstcompressor to compress the exhaust gas discharged from the compressor,and a second cooler for cooling the exhaust gas to be supplied to thesecond compressor. The respective operating efficiencies of thecompressors are increased and the efficiency of the gas turbine isincreased because the exhaust gas supplied to the compressors is cooled.The coaxially disposed compressors have one and the same shaft.

Another embodiment of the present invention includes an evaporatorcapable of recovering heat from the exhaust gas discharged from theturbine and generating steam by the recovered heat, and a reformingdevice for reforming the fuel by using steam generated by the steamgenerator and supplying the reformed fuel to the fuel cell. Thus thefuel is reformed by the steam generated by the evaporator using wasteheat of the system. When the fuel is natural gas, natural gas can bereformed to produce a fuel gas of high-quality having high CO and H₂concentrations for fuel cells.

A third embodiment of the present invention is provided with an airintake branch line through which part of air to be supplied to the fuelcell flows. When air is supplied at an excessively high flow rate to thefuel cell, part of the air is supplied through the air intake branchline to the combustor. When air is supplied at an excessively high flowrate higher than a flow rate suitable for supplying air to the fuel cellto the heat exchanger disposed above the fuel cell to cool the exhaustgas discharged from the turbine satisfactorily, excessive air is carriedby the air intake branch line to the combustor and is used for burningthe cell exhaust gas discharged from the fuel cell in the combustor.

The fuel cell-atmospheric-pressure turbine hybrid system may be providedwith a fuel supply device for supplying a fuel other than the cellexhaust gas. The combustion temperature of the cell exhaust gas can becontrolled by burning the fuel supplied by the fuel supply device in thecombustor to facilitate controlling the output of the turbine.

A fourth embodiment of the present invention includes a second turbinedisposed coaxially with the turbine as a first turbine, a secondcombustor disposed between the first and the second turbine and capableof burning a fuel and an exhaust gas discharged from the second turbineand of supplying a combustion gas to the first turbine. The exhaust gasdischarged from the first turbine is supplied to the heat exchanger. Thesecond combustor burns the exhaust gas discharged from the secondturbine and supplies the high-temperature combustion gas to the firstturbine. Consequently, the output of the first turbine increases.

A fuel cell-atmospheric-pressure turbine hybrid system in a secondaspect of the present invention includes: a combustor for burning a cellexhaust gas discharged from an atmospheric-pressure, high-temperaturefuel cell; a turbine in which a combustion gas of a pressuresubstantially equal to the atmospheric pressure discharged from thecombustor expands and the pressure of the combustion gas drops to anegative pressure; a compressor for compressing an exhaust gasdischarged from the turbine to increase the pressure of the exhaust gas;and an air supply line through which air is supplied to the combustor.

The fuel cell-atmospheric-pressure turbine hybrid system in the secondaspect of the present invention, similarly to the fuelcell-atmospheric-pressure turbine hybrid system in the first aspect ofthe present invention, includes the atmospheric-pressure,high-temperature fuel cell and the atmospheric-pressure turbine incombination. Therefore, the thermal energy of the high-temperature cellexhaust gas discharged from the fuel cell can be effectively used, anyhigh pressures are not produced in the system, an additional emergencyprotective device is unnecessary, and lightweight, easy-to-processstructural and piping materials can be used to reduce the cost. Sincethe exhaust gas discharged from the turbine is supplied to the fuel cellafter the pressure of the exhaust gas has been increased by thecompressor, the thermal energy of the exhaust gas can be effectivelyused by the fuel cell without using a circulation blower or the like.The MCFC needs much CO₂ around the cathode. Since the exhaust gas has ahigh CO₂ concentration, the power generation efficiency of the fuel cellincreases. Oxygen can be supplied to the combustor at an increased rateby supplying air through the air supply line into the combustor.Consequently, the combustion efficiency of the combustor can beincreased.

Another embodiment of the present invention includes a heat exchangerfor transferring heat of an exhaust gas discharged from the turbine toan exhaust gas discharged from the compressor. The high-temperatureexhaust gas discharged from the turbine and cooled at a low temperaturethrough heat exchange in the heat exchanger and the low-temperatureexhaust gas is supplied to the inlet of the compressor. Thus the powerfor driving the compressor can be decreased and the efficiency of theturbine can be increased. The exhaust gas discharged from the compressoris heated at a high temperature by the heat of the high-temperatureexhaust gas discharged from the turbine in the heat exchanger, and thehigh-temperature exhaust gas is supplied to the fuel cell. Consequently,the power generation efficiency of the fuel cell can be furtherincreased.

A preferred embodiment of the present invention includes an air supplybranch line branched from the air supply line to supply part of airflowing through the air supply line to the fuel cell. Thus the powergeneration efficiency of the fuel cell can be increased.

Another embodiment of the present invention further includes an airdistribution valve placed at the joint of the air supply line and theair supply branch line to adjust the distribution of air to the airsupply line and the air supply branch line. Thus air can be properlysupplied to the fuel cell according to the type and capacity of the fuelcell to increase the efficiency of the fuel cell. When the fuel cell isa solid oxide fuel cell (SOFC), the cathode of the fuel cell does notneed much carbon dioxide gas, but needs much oxygen, Therefore, the airdistribution valve is operated so as to supply air at an increased flowrate to the fuel cell. When the fuel cell is a MCFC, the cathode of thefuel cell needs much carbon dioxide gas. Therefore, the air distributionvalve is operated to stop supplying air to the fuel cell or to reducethe flow rate air flowing into the fuel cell so that the carbon dioxideconcentration of the exhaust gas compressed by the compressor andsupplied to the fuel cell increases.

As apparent from the foregoing description, the fuelcell-atmospheric-pressure turbine hybrid system of the present inventioneffectively uses the thermal energy of the cell exhaust gas dischargedfrom the atmospheric-pressure, high-temperature fuel cell, does not needany emergency protective device, and enables the use of lightweight,easy-to-process structural and piping materials to reduces the cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fuel cell-atmospheric turbine hybridsystem in a first embodiment according to the present invention;

FIG. 2 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a second embodiment according to the present invention;

FIG. 3 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a third embodiment according to the present invention;

FIG. 4 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a fourth embodiment according to the present invention;

FIG. 5 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a fifth embodiment according to the present invention;

FIG. 6 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a sixth embodiment according to the present invention;

FIG. 7 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a seventh embodiment according to the presentinvention;

FIG. 8 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in an eighth embodiment according to the presentinvention; and

FIG. 9 is a block diagram of a fuel cell-atmospheric-pressure turbinehybrid system in a ninth embodiment according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1 shows a fuel cell-atmospheric-pressure turbine hybrid system in afirst embodiment according to the present invention in a block diagram.Referring to FIG. 1, the fuel cell-atmospheric-pressure turbine hybridsystem includes an atmospheric-pressure, high-temperature fuel cell 1and an atmospheric-pressure turbine APT in combination. Theatmospheric-pressure turbine APT uses a cell exhaust gas G1 of apressure substantially equal to the atmospheric pressure discharged fromthe fuel cell 1 as a fuel. The atmospheric-pressure turbine APT isprovided with a combustor 2 for burning the cell exhaust gas G1discharged from the fuel cell 1, a turbine 3 in which a combustion gasG2 discharged from the combustor expands and the pressure of thecombustion gas drops to a negative pressure, a compressor 4 driven bythe turbine 3 to increase the pressure of an exhaust gas discharged fromthe turbine 3, and a heat exchanger 5 for transferring the heat of thehigh-temperature exhaust gas G3 discharged from the turbine 3 toenvironmental, low-temperature air A to be supplied to the fuel cell 1.The atmospheric-pressure turbine APT is of a single-shaft type. Theturbine 3 and the compressor 4 has one and the same shaft 10 in common.The shaft 10 is connected to a generator 40, namely, a load on theatmospheric-pressure turbine APT. The atmospheric-pressure turbine APTmay be of a dual shaft type provided with first and second shafts, theturbine 3 and the compressor 4 may be linked by the first shaft, and theturbine 3 and the generator 40 may be linked by the second shaft.

In the first embodiment shown in FIG. 1, the fuel cell 1 is a MCFC. Thefuel cell 1 has an anode 11, a cathode 12 and an electrolyte layer 13extending between the anode 11 and the cathode 12. Carbon monoxide (CO)and hydrogen gas (H₂) generated from the fuel F of a normal pressuresupplied to the anode 11 and air A of a normal pressure supplied to thecathode 12 interact through the electrolyte layer 13 to generateelectric power. The fuel F is, for example, natural gas.

The combustor 2 burns a normal-pressure, high-temperature cell exhaustgas G1 containing unreacted gases and excess air and discharged from thefuel cell 1 and discharges an exhaust gas G2. The turbine 3 is driven bythe exhaust gas G2 received from the combustor 2. The turbine 3 drivesthe compressor 4 and the generator 40. The combustion gas G2 expands andthe pressure thereof decreases to a negative pressure as the combustiongas S2 flows through the turbine 3 and becomes a negative-pressure,intermediate-temperature exhaust gas G3. The heat exchanger 5 transfersthe heat of the exhaust gas G3 to the low-temperature air A to besupplied to the fuel cell 1. The exhaust gas G3 thus used for heatingthe air A becomes a low-temperature exhaust gas G4. The compressor 4compresses the exhaust gas G4 at the atmospheric pressure. Since theexhaust gas G4 has a low temperature, the compressor 4 is able tocompress the exhaust gas G4 efficiently, which improves the efficiencyof the atmospheric-pressure turbine APT. The air A heated at a hightemperature by the heat exchanger 5 is supplied to the cathode 12 of thefuel cell 1. Oxygen contained in the air A serves as an oxidizer topromote the chemical reactions of the components of the fuel F and,consequently, power generating efficiency is increased. An exhaust gasG5 discharged from the compressor 4 is supplied to the fuel cell 1 andis mixed with the air A.

The fuel cell 1 is of an atmospheric-pressure, high-temperature type andthe atmospheric-pressure turbine APT is of an atmospheric pressure type.Therefore, the atmospheric-pressure turbine APT is able to useeffectively the thermal energy of the high-temperature cell exhaust gasG1 discharged from the fuel cell 1. Any high pressures are not developedin the system, any emergency protective device, which is indispensableto the conventional fuel cell-turbine hybrid system, is not necessary.Therefore, lightweight, easy-to-process structural and piping materialscan be used for constructing the fuel cell 1 and thebathometric-temperature turbine APT and thereby the system can bemanufactured at a low cost.

The exhaust gas G5 discharged from the compressor 4 is mixed in the airA to be supplied to the fuel cell 1. Therefore, the exhaust gas G5 canbe supplied to the atmospheric-pressure fuel cell by a blower or thelike requiring very low power, while the exhaust gas G5 is supplied to ahigh-pressure fuel cell by a recycle blower in the conventional hybridsystem. Consequently, the partial pressure of carbon dioxide around thecathode 12 can be easily increased and power generation efficiency canbe improved even under an operating condition where cathodic reactionrate is low. The fuel cell 1 may be a SOFC. When fuel cell 1 is a SOFC,the exhaust gas G5 is not mixed in the air A and is discharged outsideof the system.

FIG. 2 shows a fuel cell-atmospheric-pressure turbine hybrid system in asecond embodiment according to the present invention. Basically, thefuel cell-atmospheric-pressure turbine hybrid system in the secondembodiment is similar to the fuel cell-atmospheric-pressure turbinehybrid system in the first embodiment shown in FIG. 1. The fuelcell-atmospheric-pressure turbine hybrid system shown in FIG. 2includes, in addition to components corresponding to those of the firstembodiment, a spray type cooling device 6. The cooling device 6 isdisposed between the compressor 4 and the heat exchanger 5. The coolingdevice 6 sprays water on an exhaust gas G4 discharged from the heatexchanger 5 to cool the exhaust gas G4. Then, a moistened exhaust gas G6is supplied to the compressor 4. The moisture contained in the exhaustgas G6 evaporates in the compressor 4 and absorbs latent heat ofvaporization from the exhaust gas G6 to cool the exhaust gas G6 at a lowtemperature. Consequently, the compressor compresses the exhaust gas G6at a high efficiency and the efficiency of the atmospheric-pressureturbine APT is improved.

FIG. 3 shows a fuel cell-atmospheric-pressure turbine hybrid system in athird embodiment according to the present invention. The fuelcell-atmospheric-pressure turbine hybrid system in the third embodimentincludes, in addition to components corresponding to those of the fuelcell atmospheric-pressure turbine hybrid system in the secondembodiment, a second compressor 41 disposed coaxially with the firstcompressor 4 corresponding to the compressor 4 of the second embodimentand having a shaft 10 in common with the first compressor 4, a firstcooling device 6 corresponding to the cooling device 6 of the secondembodiment, a spray type second cooling device 61 disposed between thefirst compressor 4 and the second compressor 41 to cool an exhaust gasG7 discharged from the first compressor 4. The second cooling device 61sprays water on the exhaust gas G7 discharged from the first compressor4 to cool the exhaust gas G7. Then, a moistened exhaust gas G8 issupplied to the second compressor 41. The moisture contained in theexhaust gas G8 evaporates in the second compressor 41 and absorbs latentheat of vaporization from the exhaust gas G8 to cool the exhaust gas G8at a low temperature. An exhaust gas G9 discharged from the secondcompressor 41 is mixed in the air A, carbon dioxide that serves as anoxygen-carrying medium in the fuel cell 1 is recovered and is suppliedto the cathode 12. Since the two cooling devices 6 and 61 cools theexhaust gases G6 and G8 flowing into the two compressors 4 and 41,respectively. Consequently, the compressors 4 and 41 compress theexhaust gases G6 and G8 at a high efficiency and the efficiency of theatmospheric-pressure turbine APT is improved. The spray type coolingdevices 6 and 61 shown in FIGS. 2 and 3, namely, direct type watersprayers, may be replaced with indirect type cooling devices internallyprovided with cooling pipes.

FIG. 4 shows a fuel cell-atmospheric-pressure turbine hybrid system in afourth embodiment according to the present invention. The fuelcell-atmospheric-pressure turbine hybrid system in the fourth embodimentincludes, in addition to components corresponding to those of the firstembodiment shown in FIG. 1, an evaporator 7 connected to the outlet ofthe turbine 3 and a reforming device 8 connected to the evaporator 7.The evaporator 7 generates steam S by heat recovered from the exhaustgas G3 discharged from the turbine 3. The reforming device 8 decomposesthe fuel F into CO and H₂ by using the steam S generated by theevaporator 7 and supplies CO and H₂ to the fuel cell 1. Thus the hybridsystem is able to reform the fuel F by using waste heat.

FIG. 5 shows a fuel cell-atmospheric-pressure turbine hybrid system in afifth embodiment according to the present invention. The fuelcell-atmospheric-pressure turbine hybrid system in the fifth embodimentincludes, in addition to components corresponding to those of the firstembodiment shown in FIG. 1, an air intake branch line 9 through whichpart of the air A to be supplied to the fuel cell 1 flows. When the airA is supplied at an excessively high flow rate to the fuel cell 1, partof the air A is supplied through the air intake branch line 9 directlyto the combustor 2. When the air A is supplied at an excessively highflow rate higher than a flow rate suitable for supplying the air A tothe fuel cell 1 to the heat exchanger 5 disposed above the fuel cell 1to cool the exhaust gas G3 discharged from the turbine 3 satisfactorily,excessive air A is carried by the air intake branch line 9 directly tothe combustor 2 and is used for burning the cell exhaust gas G1discharged from the fuel cell 1 in the combustor 2.

FIG. 6 shows a fuel cell-atmospheric-pressure turbine hybrid system in asixth embodiment according to the present invention. The fuelcell-atmospheric-pressure turbine hybrid system in the sixth embodimentincludes, in addition to components corresponding to those of the firstembodiment, a fuel supply device 20, such as a spray nozzle, forsupplying a liquid or gaseous fuel F1 to the combustor 3. The combustor2 burns the fuel F1 supplied by the fuel supply device 20 to control thecombustion temperature of the cell exhaust gas G1 dominating thetemperature of the exhaust gas G2 to be supplied to the turbine 3. Thecontrol of the temperature of the exhaust gas G2 facilitates the controlof the output of the turbine 3. For example, when the combustor 2 isunable to burn the cell exhaust gas G1 at a sufficiently hightemperature, the temperature of the exhaust gas G3 to be supplied to theturbine 3 can be increased by burning the fuel F1 supplied by the fuelsupply device 20 to the combustor 2 and burning the fuel F1 in thecombustor 2.

FIG. 7 shows a fuel cell-atmospheric-pressure turbine hybrid system in aseventh embodiment according to the present invention. The fuelcell-atmospheric-pressure turbine hybrid system in the seventhembodiment includes, in addition to components corresponding to those ofthe third embodiment shown in FIG. 3, a first turbine 3 corresponding tothe turbine 3 of the third embodiment, a second turbine 31 disposedcoaxially with the first turbine 3, and a second combustor 21 to which aliquid or gaseous fuel F2 is supplied. An exhaust gas G12 dischargedfrom the first turbine 3 is supplied to the heat exchanger 5. An exhaustgas G10 discharged from the second turbine 31 is mixed in the fuel F2and burned in the second combustor 21, and a high-temperature exhaustgas G11 is supplied to the first turbine 3. Consequently, the output ofthe first turbine 3 increases.

FIG. 8 shows a fuel cell-atmospheric-pressure turbine hybrid system inan eighth embodiment according to the present invention in a blockdiagram. The fuel cell-atmospheric-pressure turbine hybrid system in theeighth embodiment includes, similarly to the first embodiment shown inFIG. 1, a combustor (reactor) 2, a turbine 3 and a compressor 4. Theoutlet of the compressor 4 is connected to the cathode 12 of the fuelcell 1 by an exhaust gas line 50. An air supply line 51 is connected tothe combustor 2 to supply air A1 from the environment to the combustor2.

The exhaust gas G3 discharged from the turbine 3 is compressed by thecompressor 4 to raise the pressure of the exhaust gas G3. An exhaust gasG22 discharged from the compressor 4 is supplied through the exhaust gasline 50 directly to the cathode 12 of the fuel cell 1. Thus the thermalenergy of the exhaust gas G22 can be effectively used by the fuel cell 1without using any circulating blower or the like. If the fuel cell 1 isa MCFC, the cathode 12 needs a large quantity of CO₂. Since the exhaustgas G22 supplied to the fuel cell 1 contains CO₂, the power generationefficiency of the fuel cell 1 is improved. The air A1 is suppliedthrough the air supply line 51 to the combustor 2. Consequently, oxygengas is supplied at an increased flow rate to the combustor 2, theefficiency of the reaction of the exhaust gas G1 discharged from thefuel cell 1, namely, combustion efficiency, can be increased.

This embodiment is provided with a fuel reforming device 52 forreforming a fuel F to be supplied to the anode 11 of the fuel cell 1.The reforming device 52 is connected to the lower end of the cathode 12of the fuel cell 1 by an exhaust gas branch line 53. Part G20 of thehigh-temperature exhaust gas G1 to be supplied from the cathode 12 tothe combustor 2 is supplied to the reforming device 52 and is used forreforming the fuel F.

This embodiment is provided with an air preheater 54 for preheating theair A1 that flows through the air supply line 51. The air preheater 54is connected to the fuel reforming device 52 by an exhaust gas line 55to preheat the air A1 by the heat of the exhaust gas G21 discharged fromthe fuel reforming device 52 through heat exchange in the air preheater54. Thus the preheated air A1 is supplied through the air supply line 51to the combustor 2. Consequently, the combustion efficiency of thecombustor 2 is further improved. If the fuel F is natural gas, the fuelgas F1 is decomposed into CO and H₂ by an exhaust gas G20 dischargedfrom the cathode 12, and CO and H₂ are supplied to the fuel cell 1.

An exhaust heat exchanger 56 makes an exhaust gas G5 discharged from thecompressor 4 and an exhaust gas G3 discharged from the turbine 3exchange heat. The temperature of the high-temperature exhaust gas G3discharged from the turbine 3 is decreased through heat exchange in theheat exchanger 56. A spray type cooling device 57 is disposed betweenthe exhaust heat exchanger 56 and the compressor 4. An exhaust gas G4 iscooled by the cooling device 57 and the temperature thereof decreases.The low-temperature exhaust gas G4 is supplied to the compressor 4.Consequently, power necessary for driving the compressor 4 can bereduced. The exhaust gas G5 discharged from the compressor 4 is heatedby the heat of the high-temperature exhaust gas G3 discharged from theturbine 3 in the exhaust heat exchanger 56. The high-temperature exhaustgas G22 is supplied to the cathode 12 of the fuel cell 1, which furtherincreases the power generation efficiency of the fuel cell 1. Part ofthe exhaust gas G22 being supplied from the exhaust heat exchanger 56 tothe cathode 12 may be sent to the fuel reforming device 52 to use thethermal energy of the part of the exhaust gas 22 for reforming the fuelF.

The change of state of the exhaust gases discharged from the hybridsystem shown in FIG. 8 will be numerically explained. In the followingdescription, the unit of pressures P is bar, the unit of temperatures Tis ° C. and the unit of flow rates G is kg/h. Cell output is 250 kW andplant output is 300 kW. The fuel (1.08P, 32.0T, 120G) is reformed by thefuel reforming device 52 and the reformed fuel F is supplied to theanode 11 of the fuel cell 1 to generate power by the fuel cell 1. Theexhaust gas G1 a discharged from the anode 11 and the exhaust gas G1 cdischarged from the cathode 12 are supplied to and burned by thecombustor 2. The fuel F supplied from the fuel reforming device 52 tothe anode 11 has 106P, 580T and 120G, the exhaust gas G1 a from theanode 11 has 1.05P, 650T and 620G, and the exhaust gas G1 c from thecathode 12 has 1.05P, 650T and 2500G. The exhaust gases G1 a and G1 crespectively from the anode 11 and the cathode 12 are supplied to thecombustor 2. Part of the exhaust gas G1 c from the cathode 12 issupplied through the exhaust gas branch line 53 to the fuel reformingdevice 52 to use the thermal energy of the part of the exhaust gas G1 cfor reforming the fuel F. The exhaust gas G1 c supplied to the combustor2 has 1.04P, 650T and 1180G. The exhaust gas 20 supplied to the fuelreforming device 52 has 1.04P, 650T and 1320G. The exhaust gas G3supplied from the combustor 2 to the turbine 3 has 0.99), 820T and3000G. The exhaust gas G3 discharged from the turbine 3 has 0.33P, 600Tand 3000G. The exhaust gas G4 supplied from the exhaust heat exchanger56 through the cooling device 57 to the compressor 4 has 0.31P, 40.0Tand 3000G. The exhaust gas G22 discharged from the compressor 4, heatedby the exhaust heat exchanger 56 and supplied through the exhaust gasline 50 to the cathode 12 has 1.06P, 580T and 3000G.

The air A1 taken into the air preheater 54 has 1.01P, 25.0T and 1200G.The exhaust gas G21 passing through the fuel reforming device 52 andheating the air A1 has 1.02P, 414T and 1414G. The preheated air Alflowing through the air supply line 51 into the combustor 2 has 1.00P,400T and 1200G. The output power of the fuel cell 1 is 250 kW, the powergeneration efficiency of the fuel cell 1 is 48.0% LHV. The output powerof the hybrid system is 300 kW, the power generation efficiency of thehybrid system is 57.6% LHV. Thus a high-power, high-efficiency powergenerating system is provided.

FIG. 9 shows a fuel cell-atmospheric-pressure turbine hybrid system in aninth embodiment according to the present invention. A fuel cell 1included in the fuel cell-atmospheric-pressure turbine hybrid system inthe ninth embodiment is a SOFC. The hybrid system in the ninthembodiment includes, in addition to components corresponding to those ofthe hybrid system in the eighth embodiment, an air supply line 58branching out from the air supply line 51 and connected to the cathode12 of the fuel cell 1. Part of the air A1 is supplied through the airsupply line 51 to the cathode 12 of the fuel cell 1. An air distributionvalve 59 is placed at the joint of the air supply line 51 and 58 toadjust the distribution of the air A1 to the air supply lines 51 and 58.

Part of the air A1 flowing through the air supply line 51 is suppliedthrough the air supply line 58 to the cathode 12 of the fuel cell 1. Thecathode 12 of the fuel cell 1, namely, the SOFC, does not need much CO₂,but needs much O₂. Power generation efficiency is increased byincreasing the supply of O₂ to the cathode 12 of the fuel cell 1. Theair distribution valve 59 adjust the distribution of air to the fuelcell 1 according to the capacity of the fuel cell 1 to supply the air ata proper flow rate to the fuel cell so that the efficiency of the fuelcell 1 may be improved. Distribution of the air can be adjustedaccording to the type of the fuel cell 1. For example, when the fuelcell 1 is a MCFC requiring much CO₂ for efficient power generation, theair distribution valve 59 is operated to stop or decrease the supply ofthe air A1 to the fuel cell 1 so that the exhaust gas G22 compressed bythe compressor 44 and flowing through the exhaust gas line 50 has a highCO₂ concentration. When the fuel cell 1 is a SOFC, the air distributionvalve 59 is operated to increase the supply of the air A1 to the fuelcell 1. The air distribution valve 59 is not necessarily indispensable.For example, the respective flow rates of the air flowing through apart, extending from the air distribution valve 59, of the air supplyline 51 and the air flowing through the air supply line 58 connected tothe fuel cell 1 may be adjusted by forming the part, extending from theair distribution valve 59, of the air supply line 51 and the air supplyline 58 by pipes of different nominal diameters, respectively.

Although the turbine 3 and the compressor 4 are disposed coaxially ineach of the foregoing embodiments, the turbine 3 and the compressor 4 donot necessarily need to be connected by the shaft; the compressor 4 maybe driven individually by a motor or the like.

1. A fuel cell-atmospheric-pressure turbine hybrid system comprising: acombustor configured to burn a cell exhaust gas discharged from anatmospheric-pressure, high-temperature fuel cell, theatmospheric-pressure, high-temperature fuel cell to which an atmosphericpressure air and an atmospheric pressure fuel are supplied at anatmospheric pressure and from which the cell exhaust gas is dischargedat the atmospheric pressure; a turbine in which a combustion gasdischarged at the atmospheric pressure from the combustor expands andthe pressure of the combustion gas drops to a negative pressure lowerthan the atmospheric pressure, the turbine being configured to dischargea turbine exhaust gas at the negative pressure; a compressor configuredto compress the turbine exhaust gas discharged from the turbine toincrease the pressure of the turbine exhaust gas to the atmosphericpressure and to discharge a compressor exhaust gas at the atmosphericpressure; and a heat exchanger configured to transfer heat from theturbine temperature exhaust gas discharged from the turbine to theatmospheric pressure air to be supplied to the fuel cell; an evaporatorcapable of recovering heat from the turbine exhaust gas discharged fromthe turbine and generating steam by the recovered heat; and a reformingdevice configured to reform the atmospheric pressure fuel by using steamgenerated by the evaporator and to supply the reformed fuel to the fuelcell.
 2. The fuel cell-atmospheric-pressure turbine hybrid systemaccording to claim 1, wherein the compressor exhaust gas discharged fromthe compressor is mixed in the atmospheric pressure air to be suppliedto the fuel cell.
 3. The fuel cell-atmospheric-pressure turbine hybridsystem according to claim 1 further comprising a cooling device disposedbelow the heat exchanger and configured to cool the turbine exhaust gasdischarged from the heat exchanger.
 4. The fuelcell-atmospheric-pressure turbine hybrid system according to claim 3,wherein the compressor comprises a first compressor and a secondcompressor disposed coaxially with the first compressor, and the systemfurther comprises a second cooling device disposed between the firstcompressor and the second compressor, the first compressor beingconfigured to compress the turbine exhaust gas discharged from theturbine to increase the pressure of the turbine exhaust gas and todischarge a first compressor exhaust gas, the second cooling devicebeing configured to cool the first compressor exhaust gas dischargedfrom the first compressor, the second compressor being configured tocompress the first compressor exhaust gas from the second cooling deviceto increase the pressure of the first compressor exhaust gas and todischarge the second compressor exhaust gas at the atmospheric pressureas the compressor exhaust gas.
 5. The fuel cell-atmospheric-pressureturbine hybrid system according to claim 1 wherein an air intake branchline through which part of the atmospheric pressure air to be suppliedto the fuel cell is supplied to the combustor.
 6. The fuelcell-atmospheric-pressure turbine hybrid system according to claim 1further comprising a fuel supply device configured to supply a fuelother than the cell exhaust gas to the combustor.
 7. The fuelcell-atmospheric-pressure turbine hybrid system according to claim 1,wherein the combustor has a first combustor and a second combustor, thefirst combustor being configured to burn the cell exhaust gas dischargedfrom the atmospheric-pressure, high-temperature fuel cell and todischarge the combustion gas at the atmospheric pressure, the turbinehas a first turbine and a second turbine disposed coaxially with thefirst turbine, in the second turbine the combustion gas discharged fromthe first combustor expands and the pressure of the combustion gas dropsto a first negative pressure lower than the atmospheric pressure, thesecond combustor being capable of burning a fuel and the exhaust gasdischarged from the second turbine and being configured to discharge asecond combustion gas to the first turbine, the first turbine in whichthe second combustion gas discharged from the second combustion expandsand the pressure of the second combustion gas drops to the negativepressure, the first turbine being configured to discharge the turbineexhaust gas at the negative pressure, the compressor compresses theturbine exhaust gas discharged from the first turbine to increase thepressure of the turbine exhaust gas to the atmospheric pressure.
 8. Afuel cell-atmospheric-pressure turbine hybrid system comprising: acombustor configured to bum a cell exhaust gas discharged from anatmospheric-pressure, high-temperature fuel cell, theatmospheric-pressure, high-temperature fuel cell to which an atmosphericpressure air and an atmospheric pressure fuel are supplied at anatmospheric pressure and from which the cell exhaust gas is dischargedat the atmospheric pressure; a turbine in which a combustion gas of apressure substantially equal to the atmospheric pressure discharged fromthe combustor expands and the pressure of the combustion gas drops to anegative pressure lower than the atmospheric pressure, the turbine beingconfigured to discharge a turbine exhaust gas at the negative pressure;a compressor configured to compress the turbine exhaust gas dischargedfrom the turbine to increase the pressure of the turbine exhaust gas tothe atmospheric pressure and to discharge a compressor exhaust gas atthe atmospheric pressure; an air supply line through which air at theatmospheric pressure is supplied to the combustor; an evaporator capableof recovering heat from the turbine exhaust gas discharged from theturbine and generating steam by the recovered heat, and a reformingdevice configured to reform the atmospheric pressure fuel by using steamgenerated by the evaporator and to supply the reformed fuel to the fuelcell.
 9. The fuel cell-atmospheric-pressure turbine hybrid systemaccording to claim 8 further comprising a heat exchanger configured totransfer heat of the turbine exhaust gas discharged from the turbine tothe compressor exhaust gas discharged from the compressor.
 10. The fuelcell-atmospheric-pressure turbine hybrid system according to claim 8further comprising an air supply branch line branched from the airsupply line and configured to supply part of air flowing through the airsupply line to the fuel cell.
 11. The fuel cell-atmospheric-pressureturbine hybrid system according to claim 10 further comprising an airdistribution valve placed at the joint of the air supply line and theair supply branch line and configured to adjust the distribution of airto the air supply line and the air supply branch line.